The present invention relates to a semiconductor device having the feature of rapid removal of majority carriers from the active base region thereof during turn-off of the device, and, further, to a method of fabricating such a device.
A typical prior art semiconductor device is shown in FIG. 1, illustrating a portion of a thyristor 10. The thyristor 10 comprises a lightly doped, N-type, or "N.sup.- ", bulk wafer 11, into which P.sup.+ emitter regions 12, as well as a P base region 14 and N.sup.+ emitter regions 15 have been diffused. The remaining N.sup.- region 17 comprises an active base region. The thyristor 10 is a three terminal device, having an anode 18, a cathode 20, and a gate 21; and the operation thereof is well known in the art.
At interface regions 22 of the thyristor 10, the anode 18, which usually is metallic, makes direct electrical contact with the N.sup.- active base region 17. Such a feature is known in the art as an "anode short". It is known in the art that the metal-semiconductor interface regions 22 each typically have the characteristic of a high carrier recombination velocity. Thus when turn-off of the thyristor 10 is initiated, majority carriers in the N.sup.- active base region 17 (electrons, here) are able to rapidly recombine with holes at the interface regions 22, thereby achieving rapid removal of the majority carriers from the N.sup.- active base region 17. The significance of this is that the time required for turn-off of the thyristor 10 is reduced.
In the thyristor 10, however, the width of the N.sup.- active base region 17(that is, the vertical dimension thereof in FIG. 1) is quite large, relatively speaking, and typically is about 10 mils. Having such a large active base region width enables the thyristor 10 to properly operate with, or block, relatively large voltages between the anode 18 and cathode 20, such voltages being, for example, in the order of 2000 volts. Often it occurs that a semiconductor device only needs to block a relatively low voltage, such as, by way of example, 1000 volts. The active base region width of such a low voltage device optimally is much narrower than the N.sup.- active base region width of the thyristor 10, in order to reduce the voltage drop between the anode 18 and the cathode 20 during forward conduction of the thyristor 10, and, also, to reduce the required amount of semiconductor material, and, by way of example, such width can be in the order of 100 microns.
It is not practical to fabricate such low voltage semiconductor devices with narrow active base region widths solely from bulk wafers, as is the case with the thyristor 10, above, due to the inherent problems that would arise from the use of an extremely thin and thus fragile bulk wafer. Accordingly, as is known in the art, in order to fabricate such semiconductor device with narrow active base regions, it is necessary to epitaxially grow an active base region on a substrate which comprises a bulk wafer and whch constitutes a highly doped region,corresponding to the P.sup.+ emitter regions 12 of the above thyristor 10. However, it is not possible,at least using diffusion technology, to fabricate such devices which incorporate metal-semiconductor interface regions between the active base region and the anode of the device, as is the case with regions 22 of the thyristor 10. The same problem applies to prior art thyristor 24 of FIG. 2 illustrating a modification of the thyristor 10, above. The thyristor 24 has an active base region comprising a lightly doped N.sup.- portion 25 and a more heavily doped N portion 27. As is known in the art, the presence of the more heavily doped N portion 27 between the N.sup.- portion 25 and P.sup.+ emitter regions 28 enables the width of the active base region of the device to be reduced.